.\" Copyright (C) 2004 Andries Brouwer (aeb@cwi.nl)
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH SET_TID_ADDRESS 2 2021-06-20 "Linux" "Linux Programmer's Manual"
.SH NAME
set_tid_address \- set pointer to thread ID
.SH SYNOPSIS
.nf
.BR "#include <sys/syscall.h>" "      /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.PP
.BI "pid_t syscall(SYS_set_tid_address, int *" tidptr );
.fi
.PP
.IR Note :
glibc provides no wrapper for
.BR set_tid_address (),
necessitating the use of
.BR syscall (2).
.SH DESCRIPTION
For each thread, the kernel maintains two attributes (addresses) called
.I set_child_tid
and
.IR clear_child_tid .
These two attributes contain the value NULL by default.
.TP
.I set_child_tid
If a thread is started using
.BR clone (2)
with the
.B CLONE_CHILD_SETTID
flag,
.I set_child_tid
is set to the value passed in the
.I ctid
argument of that system call.
.IP
When
.I set_child_tid
is set, the very first thing the new thread does
is to write its thread ID at this address.
.TP
.I clear_child_tid
If a thread is started using
.BR clone (2)
with the
.B CLONE_CHILD_CLEARTID
flag,
.I clear_child_tid
is set to the value passed in the
.I ctid
argument of that system call.
.PP
The system call
.BR set_tid_address ()
sets the
.I clear_child_tid
value for the calling thread to
.IR tidptr .
.PP
When a thread whose
.I clear_child_tid
is not NULL terminates, then,
if the thread is sharing memory with other threads,
then 0 is written at the address specified in
.I clear_child_tid
and the kernel performs the following operation:
.PP
    futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);
.PP
The effect of this operation is to wake a single thread that
is performing a futex wait on the memory location.
Errors from the futex wake operation are ignored.
.SH RETURN VALUE
.BR set_tid_address ()
always returns the caller's thread ID.
.SH ERRORS
.BR set_tid_address ()
always succeeds.
.SH VERSIONS
This call is present since Linux 2.5.48.
Details as given here are valid since Linux 2.5.49.
.SH CONFORMING TO
This system call is Linux-specific.
.SH SEE ALSO
.BR clone (2),
.BR futex (2),
.BR gettid (2)
.SH COLOPHON
This page is part of release 5.13 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.
